Add support for source switching

[loonycyborg]

Checklist

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Suggestion

As per #13 sometimes same .manifest file needs to support both release build and CI build. However those require entirely different source tag in .manifest while rest of it is exactly same. So projects have to patch the release manifest to point to CI sources which is bad from maintainability standpoint because you'd need to remember to check all dependent patches each time you update the manifest.

So the suggestion is to add some feature to have several alternate source tags for same module and ability to switch between them via command line flag to flatpak-builder. It would eliminate need for patching.

This feature request would probably share a lot of implementation with #3 but is driven by a different use-case.